1. Field of the Invention
The present invention relates to apparatus for gauging or measuring certain physical properties of the threading placed on the ends of tubular devices, and, more particularly, to an apparatus for measuring the pitch diameter of threaded pipe ends for determining the taper of threads on the inner or outer sides of the ends of tubular members.
2. General Background
In the oil and gas production industry, string of tubing sections of threaded pipe are joined end-to-end, i.e., pin to box end, to convey fluids. It is desirable that the joints be strong and of tight fit although not excessively tight. To obtain proper mating, pitch diameter and taper of the threading of the pin and box ends must be standardized. Such standard specifications for pitch diamter, taper and "stand-off" (the distance from the end of the threaded pipe to the plane for measuring the pitch diameter) are promulgated by the American Petroleum Institute (API). To check for conformity, gauging procedures have been standardized and various gauging devices exist for use in field locations. Some prior art patents illustrating such gauges and gauging techniques are disclosed in the following U.S. Patents:
______________________________________ U.S. Pat. No. Inventor ______________________________________ 4,189,843 Baldwin 2,874,475 McGaffey 3,432,935 Reish 2,852,856 Cox 2,668,359 Litchfield 1,950,912 Aronson ______________________________________
Such a pipe thread gauge permitting field measurement of pitch diameter and taper of pipe threads for sorting drill pipe to match pin and box ends carried on pipe so that properly interfitting joints are realized includes an elongated scale rail having a pair of blade block assemblies slidably mounted on the rail and thus movable relative to each other and a pair of thread engaging blades pivotally supported on the blade blocks to adjust the angle at which the blades extend relative to each other is described in McGaffey. This gauge, while it can measure the taper of a thread course on the end of a pipe and the end diameter, cannot measure the pitch diameter of the thread and thus cannot measure the "stand-off" between the pin and box ends, important to the joint's tightness.
Baldwin provides a thread gauge of the type describe in McGaffey, but, capable of gauging the pitch diameter and taper of threads by further providing locking and adjusting elements in combination with the slidable blade blocks for locking the blades in fixed spacial relationship to each other along the length of the rail and a graduated scale bar and micrometer assembly on the rail for setting the distance which separates the blades so that a predetermined pitch diameter can be identified on a threaded pipe by placing the blades in contact with the crest of the threads over the length of the taper at opposite sides of the pipe end which carries the thread. Baldwin further provides a series of interchangeable blade stop plates for setting the blades at preselected angles to each other to correspond to specific thread taper dimensions to be gauged.
Also, in applicant's co-pending U.S. application Ser. No. 430,586, filed Sept. 30, 1982, entitled "Method Of Field Inspecting Pin And Box Ends Of Oil Field Casings And Pipes And Simultaneously Recording Inspection Results In A Permanent Medium", a digital LED readout means to display caliper measurements of pitch diameter, out-of-round, and taper of both pin and box threaded ends of pipe and casing sections is disclosed.